1. Field of the Invention
This invention relates generally to flocculators as used in the treatment of water and wastewater and which are provided with improved means to control the mixing intensity for flocculation.
More specifically, this invention relates to hydraulic flocculators wherein flocs are formed by the passage of water through a bed of buoyant coarse media and wherein the control means for altering the mixing intensity for flocculation includes a media restrainer capable of exhibiting a variable cross-sectional area within a vertically tapered chamber. In this manner, the vertical and lateral dimensions of the media bed are respectively altered so that the mixing intensity can be varied independent of the flow rate through the flocculator.
2. Description of the Prior Art
Flocculation is the process of gentle and prolonged agitation during which coagulated particles suspended in a liquid, such as water or wastewater, coalesce in to larger masses which are removed by subsequent treatment processes, particularly sedimentation.
There are two classes of flocculators: mechanical flocculators and hydraulic flocculators. In mechanical flocculators, mechanical means are used to agitate the liquid. The speed of mechanically operated paddles or turbines as used in mechanical flocculators can be adjusted to suit variations in flow, temperature, or quality of the untreated liquid independent of the rate of flow through the flocculator. Mechanical flocculators, however, are often subject to flow short-circuiting, i.e., a portion of the incoming liquid passes through the flocculation chamber in less than the nominal detention period. Consequently, this portion of the liquid is not fully exposed to the agitation imparted by the flocculator paddles or turbines.
In hydraulic flocculators, agitation is induced by the flow of liquid through the flocculator. Hydraulic flocculators are generally simpler and less expensive than mechanical flocculators because they do not require complicated mechanical equipment. In places where it is feasible to use gravity to bring the untreated liquid to the flocculator for treatment, hydraulic flocculators do not need an outside source of power such as raw water pumping to provide the energy input necessary for flocculation. In the past, however, hydraulic flocculators have exhibited several disadvantages which have limited their use in water and waste-water treatment plants. These include: (1) very limited flexibility to respond to changes in the quality of the raw water or wastewater; (2) their hydraulic and consequent flocculation parameters are a function of flow and cannot be adjusted independently; (3) energy (or head) loss across the flocculator is appreciable and often dictates that additional pumping capacity be installed upstream from the flocculator unit; and (4) cleaning can be difficult.
Hydraulic flocculators make use of baffled channels, hydraulic jet-action, or a bed of coarse media to induce mixing for flocculation. In coarse media flocculators, agitation of the liquid is created by the passage thereof through a bed of coarse media, such as gravel.
Coarse media flocculators offer several unique advantages over mechanical and other types of hydraulic flocculators: (1) the coarse media provide ideal conditions for floc formation because of the continuous and uniform particle recontacts provided by the sinuous flow of liquid through the interstices (i.e., void spaces) formed by the media; (2) agglomerated flocs are stored within the coarse media bed, reducing sludge-storage requirements in subsequent treatment units; and (3) flocculation time can also be reduced considerably in coarse media flocculators, to as little as one-sixth of the time required by mechanical flocculators, because the entire bed is effective in the formation of sizable flocs--there is very little short-circuiting of flow through the chamber, and very few dead spaces which do not receive adequate mixing.
It is known in the prior art that tapered velocity gradients in a coarse media flocculator can be achieved by two methods, which may be used alone or in combination. In the first method, a graduated flocculation chamber is used, i.e., one in which the cross-sectional area of the chamber increases in increments. In the second method, several layers of coarse media are used, each comprising media of a different size and graded from small to large in the direction of flow.
However, in the course of using either of the above methods, the velocity gradients increase in increments, rather than continuously. In addition, the velocity gradients are still a function of flow and cannot be readily adjusted independent of the rate of flow. Furthermore, gravel-bed flocculators are susceptable to fouling, either by intercepted flocs or by biological growth in the gravel. Cleaning of the media bed can be difficult due to its densely-packed nature and because expansion of the media bed by backwashing is not feasible due to the weight and large size of the media. Therefore, gravel-bed flocculators must be cleaned manually or by means of complex and expensive backwashing and/or air scouring equipment.
No prior patents are known of flocculators making use of buoyant coarse media and with a displaceable media restrainer capable of providing a variable effective cross-sectional area. There are, however, patents covering filters utilizing buoyant media. An example will be found in U.S. Pat. No. 468,984 Boeing, which discloses an upflow filter having a bed of buoyant media comprising wood blocks restrained by a horizontal partition. Other examples will be found in U.S. Pat. Nos. 4,115,266 to Ohshima and 4,608,181 to Hsiung et al., as well as British Patent No. 1,147,054 to Miks. None of these prior art devices is seen to suggest the unique combination as called for in the instant invention.